Metal-air batteries are very attractive power sources in light of their potential high energy density. In this type of cells the oxidizing reactant (oxygen) which undergoes reduction during discharge is supplied from outside the battery. This reaction of oxygen reduction occurs in the presence of water and gives hydroxide ions (OH). The electrolyte generally used in such batteries is an alkaline electrolyte, i.e. an aqueous solution of a basic pH. The anode in metal-air batteries may undergo chemical reactions with the alkaline electrolyte as will be exemplified below for a metal-air cell comprising an aluminum anode.
Aluminum air battery is a metal air battery where the anode comprises aluminum. Aluminum is a lightweight metal, which produces three electrons per atom at oxidation. The electrochemical capacity of aluminum is 2.98 Ah/g, which is comparable to that of lithium (3.86 Ah/g). Moreover, flat aluminum anodes are not readily flammable in air atmosphere and are relatively non-expensive.
The use of aluminum as an anode, in combination with an air cathode, and a circulating highly-conductive aqueous alkali electrolyte provides a very attractive battery performance, regarding energy, power density and safety.
At normal aluminum-air battery operation conditions, aluminum dissolution in alkali electrolyte is electrochemical, according to the following reaction:4Al+3O2+6H2O→4Al(OH)3  (reaction 1)
However, in parallel to this beneficial reaction, portions of the aluminum at contact with the alkaline electrolyte undergoes the undesirable direct chemical dissolution:2Al+6H2O+2KOH→2K[Al(OH)4]+3H2↑  (reaction 2)
The ratio between the rate of the beneficial electrochemical reaction 1 to the total rate of aluminum dissolution (Reaction 1 and 2 together) produces the actual aluminum utilization efficiency coefficient [e], which is one of the major parameters characterizing the performance of an Al-air battery:
  e  =            R      ⁢                          ⁢      1                      R        ⁢                                  ⁢        1            +              R        ⁢                                  ⁢        2            
By balancing the operation parameters, such as the current density and the working temperature, and by application of certain additives, the efficiency of aluminum conversion (c) to electricity can be kept well above 90% (sometimes close to 100%).
A substantial practical obstacle for widespread implementation of Al-air batteries in practical applications such as electric vehicles, results from the requirement for such a battery to be shutdown at any moment, to be safe at standby for any period of time, and to be ready for quick restart to the full power at any moment. The main problem with those requirements is the susceptibility of aluminum to a very intensive corrosion in alkaline electrolyte. A major negative outcome of this process is that products of this process, (metal oxides/hydroxides) form a deposit layer on the anode surface. This layer passivates the anode and affects normal anode operation at the next battery operation cycle.
This process results in consumption of the aluminum anode material without generation of external electrical energy. It also results in unwanted, extensive hydrogen evolution (reaction 2), and in electrolyte degradation. Hydrogen evolution from aluminum corrosion (aluminum oxidation) in alkali solutions adds an additional safety problem to the battery halting issue.
The most straightforward way to avoid aluminum-electrolyte reaction (reaction 2) when electric load is not applied (at temporary stop or at shutdown for prolonged time) is to prevent physical contact between the aluminum electrodes and the electrolyte.
Therefore, the obligatory condition when stopping the battery is to take the electrolyte out of the cell. In the case of Al-air battery with recycling electrolyte this operation can be easily performed by re-directing (e.g. pumping) the electrolyte flow back into an electrolyte storage tank for complete battery emptying.
However, even the most thorough emptying of electrolyte from a battery (whether it is free-flow gravitational or forced by a pump) leaves some amount of electrolyte in the battery. Residual electrolyte in the battery can be found as a film on the aluminum surface. It can also be found on the cell walls, or as a liquid soaked in the porous air electrode body and entrapped in poorly-drainable corners.
Electrolyte residue that is located in direct contact with the anode will continue to react with aluminum (according to reaction 2), causing aluminum oxidation and decomposition of water from the alkaline electrolyte, and formation of a layer of aluminum hydroxide (and/or other products) on the anode surface. Moreover, after most of the water of the residual electrolyte film on the anode is consumed, the corrosion reaction does not stop. The reaction of surface film formation, can continue to a rather high extent due to three factors:
1. reaction continues because the aluminum hydroxide layer which is formed on the anode surface, is not dense, and does not prevent the reaction progress (corrosion continuation deeper into the aluminum metal body);
2. even after electrolyte at direct contact with the anode surface is consumed-reaction continues because new portions of residual electrolyte in the battery are attracted to aluminum, by capillary forces and as a result of the good wetting properties of the concentrated alkali solution.
3. water consumption by the corrosion process causes an increased local alkali concentration in the electrolyte film on the anode surface. Highly concentrated alkali solution is hygroscopic, and effectively absorbs the moisture from the ambient air. Such moisture ingress in the battery sustains further corrosion of the anode.
Unavoidable reaction of aluminum anodes with electrolyte residue entrapped in the battery is extremely damaging. First because of inert surface film formation of aluminum hydroxide on the surface of the anodes. This passivation layer results in problematic battery restart after a shutdown/standby cycle. Second, the electrolyte and reaction products can dry out thus blocking (clogging) the hydraulic system. In this case a restart of the battery will be very difficult if possible at all.
Thus, electrolyte pumping-out may not be enough to provide effective battery stop and conservation for dry long term standby without electrolyte. Currently, very careful water rinsing of the electrodes and of the system is needed in order not to leave any noticeable residual electrolyte and/or reaction product inside the system.
In order to reach this goal, i.e. to wash out all the residual electrolyte and reaction products, plenty of reserve water should be included in the battery system, thus increasing system weight and volume. This affects gravimetric and volumetric energy density of the system.
There were few attempts to solve the problem of aluminum-air battery shutdown and restart. One is described in WO 01/33659 for small single static cell Al-Air battery with a replaceable cartridge containing anode and electrolyte. In this system the shut-down-run modes of operation were carried out by emptying and then replacing the electrolyte bag. However WO 01/33659 does not disclose cleaning the cell from residual products and electrolyte. WO 2013/150520 discloses a battery shut down system wherein washing solution is used to wash and/or neutralize the electrode at shut-down. However, this method may involve substantial amounts of washing solution.